1. Field of the Invention
The present invention concerns an optical device that enables an observer to look simultaneously at what is normally in his field of vision and at a collimated image through the introduction of this image into his field of vision by means of an optical combiner including at least one semi-reflective mirror. It should be noted that, in this text, the term "collimated image" signifies an image being formed at infinity and that the fact of giving the observer a collimated image prevents him from having to adjust in different ways and, hence, saves him eye fatigue when his fixes his attention on the outside scene and fixes it on the image. The invention also concerns helmets fitted out with optical devices for the introduction of an image in the field of vision of an observer.
2. Description of the Prior Art
Such optical devices exist. They are particularly used in aeronautics to give, for example, piloting data to a pilot without his having to take his eyes off the outside scene.
Known devices have a variety of drawbacks such as optical aberrations in the devices or parabolic mirrors that work out of axis, poor image quality and bulkiness and heaviness unless fairly narrow limits are placed on the field of vision.
Thus in the document SPIE, volume 778, Display System Optics (1987), pp. 83 to 88, an article by James E. MELZER and Eric W. LARKIN entitled "An Integral Approach to Helmet Display System Design", describes an optical device that is mounted on a helmet and comprises: a generator of a synthetic light image to be collimated, an objective for collimation of the light radiation corresponding to the synthetic image and an optical combiner consisting of an afocal assembly of two parabolic mirrors, a first mirror reflecting the radiation of the collimated image towards the second mirror which is partially reflective to reflect this radiation towards the observer and, simultaneously, to enable the transmission of the radiation that comes from the exterior and is included in the field of the device. Furthermore, the optical device includes a number of deflecting mirrors to deflect the optical path and enable mounting on a helmet. In this known optical device, the parabolic mirrors work out of axis. This is a cause of aberrations and reduces the quality of the image.
This device has been improved by using a transparent strip with parallel faces, the two ends of which form the two parabolic mirrors. The collimated radiation goes through one of the parallel faces of the strip, reaches the first mirror and then the second one after several total reflections inside the strip, then comes out of the strip towards the observation pupil. This other optical device necessitates a strip with a large section and, therefore, with substantial volume and weight, unless the field of observation is reduced.